1. Field of the Invention
The present invention relates to an electronic still camera. More specifically, it relates to an electronic still camera having a charge accumulation type image pickup means furnished with a function of photographing a still image and converting it into an electric signal.
2. Description of the Prior Art
In Japanese Patent Laid-open Publication No. 194574/1984 there is proposed a still video camera which is designed to perform control of exposure to CCD (charge coupled device) based on the light measuring value prior to the exposure and gain control of an image signal of CCD. However, with such camera it was not possible to meet the case of a sudden change in the luminance of an object to be photographed during the exposure period. Further, there is proposed in Japanese Patent Laid-open Publication No. 52635/1986 a still video camera which is designed to measure during the exposure period a light equivalent to a light which is actually incident on the CCD and control the shutter speed based on the photometric value so as to obtain a proper exposure.
According to the latter system, even when the luminance of the object to be photographed shows a sudden change, proper exposure is obtainable, but it occurs for a photographer to mistake as if the exposure control were made based on the light measuring value of the luminance of the object to be photographed before exposure. Also, in case of a sudden lowering of the luminance of the object to be photographed, it may occur for the shutter speed to be lengthened beyond the limit of blurring or it may become impossible to obtain the desired shutter effect.
In general, in a lens shutter type camera using a silver salt film, an exposure error in a shutter serving also as a diaphragm usually occurs to a degree of .+-.1 Ev. In a single lens reflex focal plane shutter type camera as well, an exposure error to a degree of .+-.0.5 Ev may sometimes occur. By the way, in an electronic still video camera, because of the narrow dynamic range in the image pickup means such as CCD and its signal processing system, the exposure error must be suppressed to a range within .+-. 0.2 Ev. The same requirement applies to a case photographing in collaboration with an electronic flash unit.
However, especially in a camera of such a construction as to perform control of stoppage of flashing of the electronic flash unit based on the light measuring value, the flashing does not exactly stops at the desired flash stopping time based on the light measuring value but stops after lag of some degree, so that an over-exposure due to value in flash light amount control is produced. Accordingly, when an electronic flash unit is used, an exposure error is produced.